The present invention relates to lasers, and more particularly to a carbon dioxide slab laser.
Practical limitations arise in the operation of solid rod lasers due to the thermal gradients required to dissipate heat from the rod. A basic thermal problem common to all laser materials is that of optical distortion and birefringence. Research in heat removal techniques has lead to a variety of designs and constructions commonly known as disc, slab, and zig-zag lasers. In these developments, the approach has been to increase the cooling surface of a given volume of laser material to allow higher power dissipation densities with a lower internal temperature, and at the same time to adjust the direction of heat flow so that refractive gradients resulting from the heat flow have a minimum effect on the laser beam.
In the disc laser, a solid rod is segmented into discs which are perpendicular or at an angle to the optical axis. The individual discs are face cooled by forcing a suitable cooling fluid through the spaces between the discs. With such a design the heat flow paths are essentially parallel to the optical axis ideally resulting in no radial distortion of the laser beam. However, no significant improvement over the performance of solid rod lasers has been demonstrated with the disc laser due to problems involving stresses and optical distortions in the discs due to edge cooling effects, optical losses due to surface scattering and attenuation in the coolant, and mechanical problems associated with holding the discs in an exactly fixed position relative to each other and to the optical axis of the laser system.
Rectangular slab lasers provide a larger cooling surface and essentially a one dimensional temperature gradient across the thickness of the slab. For example, a typical NdYAG laser rod is of a cylindrical shape and water-cooled at the rod surface. The NdYAG laser rod is pumped optically using either krypton or xenon discharge lamps, and since light from these lamps is absorbed more or less uniformly within the rod, the center or axis of the rod tends to heat up. The temperature gradient from the center to the wall of the rod causes a gradation of optical property across the diameter of the rod. This gradation in turn causes difficulty with designing a good quality laser resonator. One solution to this optical gradient problem lies in the use of the slab geometry describes in references such as Koechner, "Solid State Laser Engineering", Section 7.3, pages 390-396; Chun et al "Resonant-Mode Analysis of Single-Mode Face Pumped Lasers", Applied Optics, Volume 16, No. 4, April, 1977, pages 1067-1069; and Jones et al, IEEE J. Quantum Electronics, Volume 7, pages 534- 535. The slab geometry tends to cancel the effect of the heat gradients because the laser beam zigzags in the plane of variation.
In a conventional carbon dioxide laser the discharge tube is typically 1 cm in diameter and is cooled with a water jacket. The CO.sub.2 gas is cooled by conduction to cooled outer walls. In order to enhance this cooling, high powered carbon dioxide lasers use flowing gas so that the gas as it moves along the border of a discharge tube carries heat with it. Alternate geometries provide for gas flow transverse to the discharge direction in an open geometry as described in for example Locke "Multi-kilowatt Industrial CO.sub.2 Lasers: A Survey", Industrial Applications of High Power Laser Technology, SPIE Vol. 86, 1976, pages 2-10.
Waveguide gas lasers are of the type wherein laser light proprogates through a hollow waveguide which also serves to confine the laser exciting discharge. Such lasers are described in Laakmann, U.S. Pat. No. 4,169,251; Lachambre et al "A Transversely RF-excited CO.sub.2 Waveguide Laser", Applied Physics Letters, Vol. 32, No. 10, May 15, 1978, pages 652-653; Laakmann "Transverse RF Excitation For Waveguide Lasers", Proceedings of the International Conference on Lasers, 1978, pages 741-743; Smith "A Waveguide Gas Laser", Applied Physics Letters, Vol. 19, No. 5, Sept. 1, 1971, pages 132-134; and Bridges et al, "CO.sub.2 Waveguide Lasers", Applied Physics Letters, Vol. 20, No. 10, May 15, 1972, pages 403-405. These references generally describe the radio frequency discharged pumped waveguide CO.sub.2 laser, and the direct current pumped waveguide laser. In this type of device, cooling to the walls of the waveguide is relatively efficient since the waveguide dimensions are typically only a few millimeters. The laser resonator in this type of device is generally not open as in other CO.sub.2 lasers, and the light is generally guided by the waveguide chamber. Typically, the resonator is made up by placing mirrors at each end of the waveguide. Advantageously, this type of device is compact because the waveguide is relatively small. However, the power from a sealed carbon dioxide waveguide laser is typically only 0.5 watts per centimeter length of discharge. Even though laser gas cooling and excitation are efficient, the gas volume is very small so that no net appreciable power benefit results.
In the present invention, a slab geometry has been combined with gas laser techniques to provide a structure which will generate high laser power per unit length of discharge. Additionally, conduction cooling of this structure aids in generating the high laser output power.
The geometry of the present gas slab laser, preferably a CO.sub.2 slab laser, includes a pair of cooled metal electrodes disposed parallel and in opposition to one another so that the separation of the electrode surfaces form a gap typically limited to about 5 millimeters or less in depth. The electrode surfaces are highly polished to provide a pair of highly reflective surfaces. A radio frequency discharge is provided between the electrodes suitable for creating laser action. Cooling of the gases between the electrodes is achieved by conduction to the metal surfaces of the electrodes and by flowing the gases transversely to the length of the electrodes. Unlike conventional flowing gas CO.sub.2 lasers, the electrodes of the present invention reflect and guide the laser light as it is propagated along the gap, and also serve to cool the gas by conduction.
The advantage offered by conduction cooling of the gases via the metal electrodes is complicated by the need for phase coherent single mode operation of a laser resonator. If a laser beam is to be focused to a diffraction limited spot, for surgery for example, then the beam must be phase coherent. All other beams give a larger focal spot. Typically, a resonator is formed by placing appropriate reflector mirrors at each end of the electrode geometry. For example, in a conventional CO.sub.2 laser the operation in single mode is achieved using an unstable resonator or suitable designed stable resonator. In a waveguide laser, both of the transverse dimensions of the waveguide are limited to typically less than about 3 millimeters, and plain mirrors are placed at each end of the waveguide chamber to result in single mode operation. In the waveguide laser the mode of oscillation is determined not by the resonator but by the waveguide cavity.
In contrast the slab CO.sub.2 geometry will guide the laser beam in one plane but is open in the other plane and will not guide and confine the beam. Unlike the waveguide laser the direction of propogation of the beam is determined by the resonator mirrors and not by the laser geometry. The thickness of a solid state slab is typically 1 cm so that the beam may zig zag along the slab at many different angles to the axis of the slab and each angle corresponds to a mode of propogation. The beam in such a solid state slab will hence be multimode and not phase coherent. In this CO.sub.2 slab laser the discharge slab thickness is typically 2 mm and under these conditions a single grazing angle of zig zag propogation of the laser beam along the axis of the discharge is preferred so that a single mode of propogation will dominate in the guided plane. In the unguided plane the laser beam is confined to the discharge slab by reflection from resonator mirrors placed at each end of the electrodes.
In contrast, it has been discovered that single mode operation of the present slab gas laser will result if either of two resonator structures are used. If the electrodes are approximately 1 cm or less in width, then a resonator of the stable type will result in a single mode laser output beam. For example, a plane partially transparent mirror on one end and a concave totally reflective spherical mirror on the other end will result in a single mode laser output beam. On the other hand, if the electrodes are in excess of one centimeter in width then a resonator of the unstable type is necessary. For example, a concave totally reflective spherical mirror and a convex totally reflective spherical mirror placed 30 cm apart at opposite ends of the electrodes will result in single mode operation. Additionally, if the electrodes are held apart to provide a gap of 2 millimeters and the distance between the edge of the electrode and the convex mirror is also held at about 2 millimeters, then a beam of 2 millimeters square will emerge which some distance from the laser becomes circular, i.e. a single mode operation.
The present invention thus provides a gas slab laser which will generate more gas laser power per unit length of discharge than other conduction cooled gas laser structures. Additionally, the present invention provides a laser resonator which will produce a single mode laser beam from a gas slab discharge. For applications such as surgery where size of the laser is important this is clearly an advantage.